Recently, polyacrylamide (PAM) has been added in a large amount to pulp slurry in the manufacturing process of paper board, in order to prevent the reduction of paper board strength caused by the degradation of pulp quality attributed to the increase of usage amount of recycled waste paper. The addition of PAM increases the strength of paper board, but reduces flexibility and water absorption of the paper board. Particularly, in the case of corrugated board raw paper, the addition of PAM results in the reduction of the adhesion between a corrugating medium and a liner in the process of adhering corrugated board, and results in the increase of a load of power required for corrugation. For the purpose of making paper board that has flexibility and proper water absorption, excellent tensile strength, good compressive strength and good bursting strength, and good adhesion property, starch has become important as a chemical instead of PAM.
However, the solution of the problem has become more difficult by the increase of paper machine speed and the development of a paper machine which decreases retention of starch by dehydrating from both sides of a layer of paper, such as twin-wire paper machine. As the reflection of such tendency, it has been required to improve PAM used conventionally or to develop novel starch effectively increasing paper strength.
When starch is used as a modifier in paper-making process, generally there are two methods. The first is a method comprising the steps of adding starch for wet-end, which is referred to as "wet-end addition", flowing it on a wire of the paper machine, and paper-making. The second is a method comprising the step of spraying a slurry of starch granules on a wet web on the wire of the paper machine. In the first method, the addition of starch can be conducted either by adding the ungelatinized starch in the state of slurry for the wet-end, or by adding the gelatinized starch in the state of solution for the wet-end.
In the method in which a solution of gelatinized starch is added as a wet-end additive, the paper layer is not expected to achieve high retention of starch, which has neutral or negative charge, such as the unmodified starch or a phosphorylated starch, since the starch has no attraction to negative charge of the pulp. On the other hand, cationic starch conventionally used has attraction to the pulp to provide good retention of starch in the paper layer when it is used in an amount of less than 1% based on a pulp to make paper board. However, when the conventional cationic starch is added in an amount of more than several percent, the retention of starch in the paper layer adversely reduces and the starch enters in white water to increase a load of treating waste water from a paper machine. Also, in the method in which ungelatinized starch granules are added as a wet-end additive to pulp slurry in the state of slurry, the retention of starch is lower than 50% when conventional starch is used. Further, in the method in which starch granules are sprayed in the state of slurry on the wet web on the wire to retain them on the surface of and in the paper layer, the retention of starch is not decreased when using the Fourdrinier paper machine, a tanmo paper machine and a multicylinder paper machine, etc., dehydrating the paper layer from one side, but the starch granules escape from the paper layer and are not retained therein when using a twin-wire paper machine dehydrating the paper layer from both sides of the layer. Thus, it is required to develop starch having high retention in a paper layer.
Starch is generally composed of granules having a granule size of several micrometers to about one hundred micrometers, and is not dissolved in cold water, and the starch granules added in water are present in the state of slurry suspended in the water. However, when the slurry is heated, the starch granules absorb water to start swelling. When they are continuously heated, the starch granules are lapsed at about 90.degree. C., and are dispersed in the form of molecules to form so-called gelatinized solution. Tackiness or adhesion peculiar to the starch is provided by the starch dispersed in the state of molecules.
In the process of making paper and paper board, the pulp suspended in water is dehydrated on the wire of the paper machine to form a wet web in which pulp is loosely bonded. Then, in the process of pressing and drying, the bond between pulps is more strongly dependent on the degree of removing water to form paper layer. However, paper strength is generally increased by adding a solution of gelatinized starch, in which starch granules are dispersed in the state of molecules, or adding PAM to increase the bonding strength between pulps.
When a gelatinized solution of raw starch which is electrically neutral or phosphorylated starch which is negatively charged is added as a wet-end additive, the bonding strength between starch molecules and pulp is relatively low, and most of the starch dispersed in the gelatinizated solution is removed from the paper layer together with the white water dehydrated on the wire of a paper machine. As the result, the retention of starch in the paper layer is too low to contribute enhancement of paper board strength. Further, the starch contained in the white water makes waste water treatment difficult.
When the gelatinized solution of the cationic starch is added as a wet-end additive at a low content per a pulp, it provides high bonding strength by the electrostatic attraction, and drastically increases the retention of starch. However, when the charge amount of the cationic starch added is more than the negative charge amount of the pulp, the retention of starch is adversely lowered. Accordingly, high retention of starch is accomplished when the cationic starch is added in an amount of less than about 1% per pulp as seen in paper, but retention is lowered when the starch is added in an amount of more than about 1%, for example several % as seen in improvement of paper board strength.
Another method is also known to the art, in which an aqueous suspension of starch granules is added as a wet-end additive in pulp slurry, and mechanically trapped in a wet web during dehydrating on a wire of a paper machine, whereby the starch granules are gelatinized at a heating and drying process to create adhesion power. In this method, when raw starch or negative charged starch is employed, the retention of starch increases more than the internal addition of the solution of gelatinized starch, but it is still less than 50%. Also, when a cationic starch is employed, the advantage of positive charge does not appear separated from the addition of the solution of gelatinized starch and the retention is kept less than 50%.
In a method in which paper board strength is improved by spraying a slurry of the starch granules on a surface of a wet web on a wire of a paper machine to retain the starch in the paper layer, good retention of starch is provided when using the Fourdrinear paper machine, a tanmo paper machine and a multicylinder paper machine dehydrating paper layer from one side of the paper layer. However, it is difficult to provide a uniform distribution of the starch granules in the paper layer as a density of the granules is increased near the surface of the layer of paper. The uniform distribution in the paper layer can be accomplished when a location of spraying is moved upstream of the wire where the paper layer is insufficiently formed, but the retention of starch is lowered by increasing leakage of the starch in the white water. Further, the retention of starch significantly reduces when the twin-wire paper machine dehydrating the wet web through the wires on both sides of the wet web is employed.
Although paper strength is increased by adding PAM, its amount is limited and an increase of the amount is difficult, because it causes lowering drainage property, and fouling a felt in the process of paper-making and, besides decreasing adhesion from decreasing water absorption of the paper in the process of making corrugated board and increasing a load of power required for corrugating it from decreasing the flexibility of paper board. Accordingly, it is strongly desired to develop a starch having an improved retention in the process of paper-making, but there has been no starch which has sufficient properties. It is required to develop starch which has a good retention, excellent reinforcing properties of paper board and easy treatment in an industrial production.